1 files changed, 9 insertions, 0 deletions
diff --git a/content/math/inversions.cpp b/content/math/inversions.cpp
new file mode 100644
index 0000000..9e47f9b
--- /dev/null
+++ b/content/math/inversions.cpp
@@ -0,0 +1,9 @@
+ll inversions(const vector<ll>& v) {
+	Tree<pair<ll, ll>> t; //ordered statistics tree @\sourceref{datastructures/pbds.cpp}@
+	ll res = 0;
+	for (ll i = 0; i < sz(v); i++) {
+		res += i - t.order_of_key({v[i], i});
+		t.insert({v[i], i});
+	}
+	return res;
+}